1. Field of the Invention
The present invention relates to an image processing apparatus which performs image restoration.
2. Description of the Related Art
An object shot via an image pickup optical system suffers from diffraction, aberration, and other adverse effects which occur in the image pickup optical system. These adverse effects make it impossible for light emitted from one point to converge to another point, which causes the light to be minutely spread around the another point. Such a minutely-spread distribution is referred to as point spread function (PSF). Such adverse effects which occur in the image pickup optical system cause a shot image to be formed, with the PSF being convoluted with the image. This makes the image blurry, leading to a decrease in resolution.
With the retention of a shot image as electronic data having become common in recent years, image restoration processing in which a deterioration of an image caused by an optical system is corrected by using image processing has been proposed. As the image restoration processing, for example, image restoration processing by using the Wiener filter is known. An image shot by a typical digital camera has different deterioration characteristics depending on its position (image height). In many cases, an amount of the aberration of the image pickup optical system is large and the aberration is asymmetrically deteriorated at the periphery of the shot image.
International Publication No. WO 2007/74649 discloses a system where a phase mask that modulates a wavefront is inserted near a position of an aperture of an image pickup optical system. If an image as a whole has certain deterioration characteristics and has frequency characteristics including no zero point, the image can be easily restored with the use of the Wiener filer. The system disclosed in International Publication No. WO 2007/74649, however, requires a special optical design in which a phase mask is inserted. This design makes it difficult to apply this system to a typical digital camera.
Both of International Publication WO 2006/041127 and the literature “OSA VOLUME 62, NUMBER 1 JANUARY 1972 Bayesian-Based Iterative Method of Image Restoration” disclose a method of performing blind deconvolution processing with the use of the so-called Lucy-Richardson method (the LR method). The literature “J. Opt. Soc. Am. A/Vol. 12, No. 1/January 1995 Blind deconvolution by means of the Richardson-Lucy algorithm” discloses blind deconvolution processing in which image restoration processing can be performed while estimating deterioration characteristics even if a shot image is only known information. However, if the shot image has shift-variant deterioration characteristics, it is difficult to satisfactorily restore, with conventional arts, the image in frequency regions in which an optical transfer function has a value close to zero.